Abstract
Replacing traditional TaCl5 precursors, calixarene-Ta(V) complexes are grafted on SiO2 at self-limiting loadings of 0.2 mmol.g(-1) (0.25 Ta.nm(-2)) to synthesize an active and selective catalyst for epoxidation with aqueous H2O2. The catalyst is synthesized in a one-pot procedure and has turnover rates that are a very weak function of Ta surface density, in contrast with catalysts that are subsequently calcined, following traditional practice. Cyclooctene epoxidation turnover rates exceed 240 h(-1) and epoxide hydrolysis to the cyclooctanediol is extremely low, <2%, and does not increase with conversion. Calcination decreases turnover rates by 30%. and undesired selectivity to the cyclooctanediol is 5% and increases with conversion. The background rate of unproductive H2O2 decomposition is also decreased by 50% when using the calixarene capping ligand as compared to the bare oxide. A kinetic study of this system indicates a near first order dependence of the epoxidation rate on Ta content and H2O2 concentrations: cyclooctene is weak positive order indicating some inhibition. H2O, product epoxide, and co-product cyclooctanediol are inhibitors, with cyclooctanediol being the strongest inhibitor on a molar basis, underscoring the importance of the reduced selectivity towards this species for the capped catalyst. A kinetic expression is proposed and describes initial rates and epoxide yields with time in good agreement with experimental data. The proposed catalytic cycle includes equilibrated formation of six-coordinate Ta(OH)X surface species as the most abundant intermediates. The rate limiting step is proposed to be the reaction between an activated Ta-hydroperoxide and an alkene in free solution, involving an external nucleophilic attack of the pi-system of the olefin on the relatively electropositive oxygen of the Ta-hydroperoxide species. The bulky and hydrophobic phenolate ligands pay a role in maintaining high Ta oxide dispersion, decreasing inhibition by polar species, and creating intrinsically more Lewis acidic sites, leading to increased epoxidation rates and decreased rates of undesired H2O2 decomposition and epoxide hydrolysis. This new class of ligand-capped, supported Ta oxide catalyst is not only more active and selective than its bare oxide analogue, but also provides a well-behaved catalyst for kinetic modeling. (C) 2010 Elsevier B.V. All rights reserved.